We are engaged in an attempt to understand the details of the structure and in vivo function of the SV40 non-virion protein(s). We have purified T antigen to a high degree and recently have developed a semi-functional assay for this protein based on its ability to bind to SV40 DNA. Moreover, studies with T antigen from a cell line transformed by a temperature-sensitive mutant of SV40 bearing its lesion in the A gene reveal that this molecule is thermolabile in its DNA binding properties when compared to wild type. These results strongly suggest that the A gene product is T antigen, a precursor or a derivative, and in view of the phenotype of this mutant virus, that T antigen probably plays a role in the initiation of viral DNA replication and possibly in the maintenance of transformation as well. In addition, preliminary efforts are along in the identification by restriction nuclease fragment mapping of the preferred T antigen binding sites on the SV40 genome. BIBLIOGRAPHIC REFERENCES: Livingston, D.M., Henderson, I.C., and Hudson, J. SV40 T Antigen: Partial Purification and Properties. IN: Cold Spring Harbor Symposium on Quantitative Biology, 39: 283-289, 1974. Jessel, D., Hudson, J., Landau, T., Tenen, D., and Livingston, D.M. Interaction of Partially Purified Simial Virus 40 T Antigen with Circular Viral DNA Molecules. Proc. Nat. Acad. Sci. (USA) 72: 1960-1964, 1975.